Venetian type blinds have a series of slats hung on ladders that extend from a headrail to a bottomrail. In most venetian blinds a pair of lift cords is provided each having one end attached to the bottomrail and then passing through elongated holes in the slats up to and through the headrail. When the lift cords are pulled downward the blind is raised and when the lift cords are released the blind is lowered. A cord lock is usually provided in the headrail through which the lift cords pass. The cord lock allows the user to maintain the blind in any desired position from fully raised to fully lowered. Pleated shades and roman shades are also raised and lowered by lift cords running from the bottom of the shade into a headrail. The cord lock system and other cord lift systems used in venetian blinds can also be used in pleated shades and roman shades.
Another type of lift system for window blinds utilizes a take-up tube for each lift cord. These tubes are contained on a common shaft within the headrail. Each lift cord is attached to one end of a tube. The tubes are rotated to wind or unwind the lift cord around tubes. This system is generally known as a tube lift system. Some tube lift systems are operated by a continuous loop cord that passes over one end of the axle and extends from the headrail.
In recent years the art has been concerned that cords, particularly looped cords, pose a strangulation threat to children who may become entangled in the cords. Consequently, there has been much interest in cordless blinds. These blinds rely on electric motors or spring motors to raise and lower the lift cord. One common cordless blind simply contains a motor connected to a tube collection system within the headrail. Another cordless blind relies upon a constant force spring motor attached to a spool or spools on which the lift cords are collected. This type of cordless blind is disclosed by Coslett in U.S. Pat. No. 5,105,867 and by Kuhar in U.S. Pat. Nos. 5,482,100; 5,531,257 and 6,079,471 and by Wang et al. in U.S. Pat. Nos. 6,012,506, 6,024,154 and 6,029,734.
Coslett discloses a sun shade having a series of blades connected together to form a serrated shape like a pleated shade. The upper blade is mounted within a hollow housing and the lower blade is secured to a plate member. A constant force spring plate is wound around a spring spool member and further engaged to an output spool, both of which are within a hollow handle secured to the hollow housing. A cord is connected to the output spool and passed from the handle through the housing and the blades and is connected to the plate member. Such a cording arrangement is similar to that of a lift cord in a pleated shade or venetian blind. The spring retains the blades in a folded closed position. When the shade is extended the spring exerts tension on the cord. Consequently, Coslett teaches the user to fix the plate member along one side of the window and to provide a hook to retain the hollow housing at the opposite side of the window when the shade is covering the window. Thus, Coslett's shade can be in only one of two positions, fully extended to cover the window or fully retracted. Furthermore, Coslett's blind is not suitable for installation in an orientation in which one rail is fixed at the top of the window frame as is done for most building windows. That is so because when the blind is fully retracted most people could not reach the handle to extend or close the blind without standing on a stool or ladder.
Kuhar discloses a cordless, balanced blind that contains at least one constant variable force spring motor in the headrail. The springs in these motors vary in thickness or in width along their length as they are wound around storage drums. A cord spool is coupled to one or more spring drums. The lift cords of the blind are wound about the spool. Thus, the spring winds or unwinds as the blind is raised or lowered. The difference in width or thickness of the spring compensates for the increasing weight of the blind on the cords as the window covering is raised and the decreasing weight as the blind is lowered. Kuhar teaches that much effort must be made to select and couple the spring motor to the cords so that the bottomrail is balanced at any and every position. Kuhar further teaches that several spring motors may be coupled together.
If the system is not in balance when the operator positions the bottomrail at a desired location, the bottomrail moves upward or downward to a location at which the system is balanced. Consequently, it is not possible to keep the bottomrail at the desired location without adjusting or replacing the spring motors. Several people in the industry have recognized that a solution to the problem is to provide a cord lock or brake that acts on the lift cords or spring motors. Wang et al. in U.S. Pat. No. 6,029,734 disclose a cordless blind with a locating unit provided in the bottomrail which prevents the lift cords from moving until the operator presses a button on the bottomrail. Another lock mechanism which engages the coil springs in a cordless blind is disclosed in U.S. Pat. No. 6,024,154. Both the springs and the lock mechanism are located in the bottomrail. This lock mechanism is also biased to a locked position. The bottomrail can be raised and lowered only while the lock button is being pressed to disengage the lock. Palmer in Published United States Patent Application 2002/0088562 discloses a one way brake which prohibits the bottomrail from moving toward the headrail, but permits the bottomrail to be moved away from the headrail by an operator. The brake must be released by pushing a button or lever in the bottomrail to raise the bottomrail. All of these cordless blinds require the operator to hold the lock button or lever to move or raise the bottomrail. If such a blind is installed in a tall window many people would be unable to reach a fully raised blind without climbing on a ladder or chair. Even if the blind were in a standard window, short people would not be able to fully raise the blind without using a ladder. Operators would also have difficulty fully raising such a blind if a couch or other furniture were in front of the window.